1. Field of the Invention
This invention relates to a variable valve train apparatus for an internal combustion engine (hereinafter referred to as engine).
2. Description of the Related Art
To realize optimum engine-output characteristics suitable for individual operating regions, a variety of engines have been proposed that are adapted for example to change a valve-opening period and a lift amount of intake and exhaust valves (refer to Japanese unexamined patent publication nos. 2001-14017, 2-223613, 2003-343225, 7-102921, and 10-18826, and Japanese patent no. 2700691, for instance).
As shown in plain view of FIG. 17, Japanese unexamined patent publication no. 2001-14017 discloses a variable valve train apparatus for an engine, which comprises a rocker shaft 104 on which low-speed and high-speed rocker arms 102, 103 are supported for rocking motion. These rocker arms 102, 103 are rocked by low-speed and high-speed cams 101a, 101b of a camshaft 101 through the medium of rollers 102a and 103a, respectively. An intake valve 105 is driven to open or close in conjunction with a rocking motion of the low-speed rocker arm 102. The low-speed rocker arm 102 is provided with a piston 106 disposed for sliding motion in response to oil pressure, and the high-speed rocker arm 103 has a linkage arm portion 107 whose distal end is adapted to be connected with or disconnected from the piston 106 of the low-speed rocker arm 102 according to the piston position.
When the connection between the low-speed and high-speed rocker arms 102 and 103 through the piston 106 is released, the high-speed rocker arm 103 is rocked idle (i.e., makes a rocking motion under no-load condition), and the low-speed rocker arm 102 drives the intake valve 105 to open or close along the shape of the low-speed cam 110a. When the connection between the low-speed and high-speed rocker arms 102, 103 is established, the low-speed rocker arm 102 is rocked integrally with the high-speed rocker arm 103, and the intake valve 105 is thereby driven to open or close along the shape of the high-speed cam 101b. 
As shown in a plan view of FIG. 19, Japanese unexamined patent publication no. 2-223613 discloses a variable valve train apparatus for an engine including a rocker shaft 204 on which are supported low-speed and high-speed rocker arms 202, 203 that are respectively rocked by low-speed and high-speed cams 201a, 201b of a camshaft 201. An intake valve 205 is driven to open or close in conjunction with a rocking motion of the low-speed rocker arm 202. The low-speed rocker arm 202 has a spacer member 206 disposed for sliding motion axially of the rocker shaft 204 in response to oil pressure, and the high-speed rocker arm 203 has an adjustment bolt 207 for pressing the spacer member 206 of the low-speed rocker arm 202 in conjunction with a rocking motion of the high-speed rocker arm 203. The adjustment bolt 207 is connected with or disconnected from the spacer member 206 according to the sliding position of the spacer member 206.
When the connection between the low-speed and high-speed rocker arms 202, 203 through the spacer member 206 is released, the high-speed rocker arm 203 is rocked idle, and the low-speed rocker arm 202 drives the intake valve 105 to open or close along the shape of the low-speed cam 201a. When the connection between the low-speed and high-speed rocker arms 202, 203 is established, on the other hand, the low-speed rocker arm 202 makes a rocking motion integrally with the high-speed rocker arm 203, and the intake valve 205 is driven to open or close along the shape of the high-speed cam 201b. 
In a variable valve train apparatus disclosed in Japanese unexamined patent publication no. 2003-343225, a high-speed rocker arm is supported on a rocker shaft and rocked by a high-speed cam, and a pair of low-speed rocker arms is supported on both sides of the high-speed rocker arm and rocked by a low-speed cam. With rocking motions of the low-speed rocker arms, a pair of intake valves is driven to open or close. The low-speed rocker arms are provided with pistons that are adapted for sliding motion in response to oil pressure. Linkage arm portions are integrally formed on both sides of the high-speed rocker arm to correspond to these pistons.
Each linkage arm portion of the high-speed rocker arm is connected with or disconnected from the piston of the corresponding low-speed rocker arm according to the piston position. When the connection between the low-speed and high-speed rocker arms is released, the high-speed rocker arm is rocked idle, and the low-speed rocker drives the intake valve along the shape of the low-speed cam. When the connection is established, the low-speed rocker arm is rocked integrally with the high-speed rocker arm, and the intake valve is driven to open or close along the shape of the high-speed cam.
It is conceivable that a variable valve train apparatus is not provided with the aforementioned low-speed rocker arms for separately driving a pair of intake valves to open or close, but is provided with a common low-speed rocker arm for driving these intake valves to open or close. Such a variable valve train apparatus is shown by way of example in FIG. 20 that includes low-speed and high-speed arms 301, 302 supported on a rocker shaft 303 and individually rocked by low-speed and high-speed cams of a camshaft 307 through the medium or rollers 301a, 302a. The low-speed rocker arm 301 has a boss portion 301b from which a pair of bifurcated valve arm portions 301c is extended so as to be connected with intake valves 104. The high-speed rocker arm 302 is provided with a piston 305 similar to that of the aforementioned variable valve train apparatus, and the low-speed rocker arm 301 is integrally provided with a linkage arm portion 306 to correspond to the piston 305. When the connection is released depending on the piston position, the intake valves are open or closed along the shape of the low-speed cam. When the connection is established, the intake valves are open or closed along the shape of the high-speed cam.
As shown in FIGS. 21 and 22, a variable valve train apparatus disclosed in Japanese unexamined patent publication no. 2003-343225 includes a high-speed rocker arm 402 supported on an intake rocker shaft 403 and rocked by a high-speed cain 401a of a camshaft 401 through the medium of a roller 402a, and a pair of low-speed rocker arms 404 supported on both sides of the high-speed rocker arm 402 and rocked by low-speed cams 401b of the camshaft 401 through rollers 404a. A pair of intake valves 405 is driven to open or close with rocking motions of the low-speed rocker arms 404. Pistons 406 are formed in the low-speed rocker arms 404 and adapted for sliding motion in response to oil pressure. Linkage arm portions 407 are integrally formed on both sides of the high-speed rocker arm 402 to correspond to the pistons 406.
The linkage arm portions 407 of the high-speed rocker arm 402 are connected with or disconnected from the pistons 406 according to the piston position. In a low-speed mode for an ordinary rotation speed region, the connection between the linkage arm portions 407 and the pistons 406 is released, and the high-speed rocker arm 402 is rocked idle, with the intake valves 405 driven to open or close along the shapes of low-speed cams 401b through the medium of the low-speed rocker arms 404. In a high-speed mode for a high-speed rotation region, the connection between the linkage arm portions 407 and the pistons 406 is established, and the low-speed rocker arms 404 are rocked integrally with the high-speed rocker arm.402, whereby the intake valves 405 are driven to open or close along the shape of the high-speed cam 401a. 
Further, a pair of exhaust rocker arms 409 is supported on an exhaust rocker shaft 408 that is provided on the side opposite the intake rocker shaft 403 with respect to the camshaft 401. The exhaust rocker arms 409 are always rocked by exhaust cams 401c of the camshaft 401, so that exhaust valves 410 are driven to open or close.
Japanese unexamined patent publication nos. 7-102921 and 10-18826 disclose an engine in which low-speed and high-speed rocker arms are supported on a rocker shaft and individually rocked by low-speed and high-speed cams, and a T-shaped lever is integrally formed on the low-speed rocker arm to drive a pair of intake valves to open or close. The high-speed rocker arm is connected with or disconnected from the low-speed rocker arm in response to a switching action of a switching pin. When the connection via the switching pin is released, the high-speed rocker arm is rocked idle, and the intake valves are driven to open or close following the shape of the low-speed cam via the low-speed rocker arm. When the connection via the switching pin is established, the low-speed rocker arm is rocked together with the high-speed rocker arm, so that the intake valves are driven to open or close following the shape of the high-speed cam.
In an engine disclosed in Japanese patent no. 2700691, an eccentric rocker shaft is utilized to establish or release the connection between low-speed and high-speed rocker arms, instead of using a switching pin. Specifically, the low-speed rocker arm is rocked by a low-speed cam to drive a pair of intake valves to open or close. The high-speed rocker arm is supported eccentrically to the axis of the rocker shaft, and one side thereof is in contact with the low-speed rocker arm. The high-speed rocker arm assumes a vertical position thereof adjusted according to the angle for which the rocker arm is rocked. At a lower position, the high-speed rocker arm is separated from the high-speed cam and rocked idle, and the intake valves are thereby driven to open or close along the shape of the low-speed cam, as mentioned above. At an upper position, the low-speed rocker arm is rocked by the high-speed cam together with the high-speed rocker arm, so that the intake valves are driven to open or close along the shape of the high-speed cam.
In the case of a four-valve SOHC engine to which the variable valve train apparatus disclosed in Japanese unexamined patent publication no. 7-102921 or 10-18826 or Japanese patent no. 2700691 is applied, the low-speed and high-speed rocker arms are laid out as shown by way of example in FIGS. 23 and 24. Specifically, intake and exhaust rocker shafts 502, 503 are disposed on both sides of a camshaft 501. An intake high-speed rocker arm 504 is supported for rocking motion on the intake rocker shaft 502, and a pair of intake low-speed rocker arms 505 are supported for rocking motion on both sides of the intake high-speed rocker arm 504. An outer end of each intake low-speed rocker arm 505 is connected to a corresponding one of intake valves 513a. Each of rollers 504a, 505a provided at inner ends of the rocker arms 504, 505 is in contact with a corresponding one of intake high-speed and low-speed cams 506, 507 of the camshaft 501 and adapted to make a rocking motion. Reference numeral 512 denotes a spark plug.
A pair of exhaust low-speed rocker arms 508 is supported for rocking motion on an exhaust rocker shaft 503, and a pair of exhaust high-speed rocker arms 509 is supported for rocking motion on both sides of the exhaust low-speed rocker arms 508 whose outer ends are respectively connected to exhaust valves 513b. Rollers 508a, 509a provided at inner ends of the rocker arms 508, 509 are in contact with exhaust low-speed and high speed cams 510, 511 on the camshaft 511 and adapted to make a rocking motion.
Between the intake low-speed rocker arm 505 and the intake high-speed rocker arm 504 and between the exhaust low-speed rocker arm 508 and the exhaust high-speed rocker arm 509, there are provided changeover mechanisms, not shown, each of which is constituted for example by a switching pin disclosed in Japanese unexamined patent publication nos. 7-102921 and 10-18826. As in the case of these patent publications, the connection between the low-speed and high-speed rocker arms 504, 505; 508, 509 on the intake and exhaust sides is established or released, and the intake valves 513a and the exhaust valves 513b are driven to open or close along the shapes of the low-speed cams 507, 510 or the shapes of the high-speed cams 506, 511, respectively.
In the variable valve train apparatus disclosed in Japanese unexamined patent publication no. 2001-14017, when the connection between the low-speed and high-speed rocker arms 102 and 103 through the piston 106 is established, the high-speed rocker arm 103 is applied at its roller 103a with a driving force from the high-speed cam 101b as shown by a bold arrow in FIG. 17, and acts to press the piston 106 with its linkage arm portion 107. Since a distal end of the linkage arm portion 107 is offset from the roller 103a axially of the rocker shaft 104, there occurs minute inclination of the high-speed rocker arm 103 each time it is applied at its roller 103a with the driving force from the high-speed cam 101b. As a result, misalignment is caused between the high-speed cam 101b and the roller 103a, and a deviated load is exerted on the roller 103a, as shown in FIG. 18.
Since the deviated load on the roller 103a causes deviated wear of the roller 103a and the high-speed cam 101b, reduction in durability of roller bearings, etc., the roller width must be widened as a countermeasure therefor, resulting in the increase in inertia mass of the high-speed rocker arm 103. This poses a problem that the opening and closing characteristic of the valve train is worsened, especially, in a high-speed rotation region. This problem is especially noticeable in a rocker arm provided with a roller, but a similar problem is caused also in a rocker arm using a slipper instead of a roller.
In the variable valve train mechanism disclosed in Japanese unexamined patent publication no. 2-224613, as shown in FIG. 19, the spacer member 206 of the low-speed rocker arm 202 is provided to protrude toward the high-speed rocker arm 203, so that the spacer member may be pressed by the adjustment bolt 207 on the high-speed rocker arm 203. In other words, that position (hereinafter referred to as driving-force transmission point 208) of the spacer member 206, which is pressed by the adjustment bolt 207 when the connection is established, is offset by a large amount of A41 from the connecting part 209 between the low-speed rocker arm 202 and the intake valve 205.
As a result, the driving force transmitted to the low-speed rocker arm 202 through the driving-force transmission point 208 has a component of force that is not effectively utilized for the opening of the intake valve 205 but generates a deviated load on a bearing through which the low-speed rocker arm 202 is supported on the rocker shaft 204. The deviated load on the bearing increases wear and friction, and by extension disadvantageously lowers the durability and reliability of the variable valve train apparatus. In addition, the component of the driving force is consumed to serve to bent or twist part in the vicinity of the driving-force transmission point 208 of the low-speed rocker arm 202, that is, part in the vicinity of the spacer member 206 that receives the driving force from the adjustment bolt 207. Accordingly, there periodically occurs undesired bending and torsion in the vicinity of the driving-force transmission point 208 of the low-speed rocker arm 202 each time the driving force is transmitted from the high-speed rocker arm 203, resulting in a problem of deteriorating the opening and closing characteristic of the intake valve in the high-speed rotation region or the like.
In the variable valve train apparatus shown in FIG. 20, the valve arm portion 301c of the low-speed rocker arm 301 serving to open the intake valve 304 against an urging force of the valve spring is required to have a sufficient strength and rigidity. In a case where the valve arm portion 301c is bifurcated and extended from a single point or a boss portion 301a as explained above, not only the arm length increases but also the valve arm portion 301c is subject to bending and torsion due to the reaction force from the valve spring. This is disadvantageous in strength and rigidity. To ensure the strength and rigidity, the weight of the valve arm portion 301c undesirably increases. This causes a valve jump and bounce especially in high-speed rotation region, posing a problem that the opening and closing characteristic of the valve train is worsened.
The variable valve train apparatus of this type requires a wide installation space as compared to the ordinary one. Particularly in a case where the variable valve train apparatus shown in FIG. 20 is mounted to both the intake and exhaust sides of a single camshaft, these variable valve train apparatuses occupy a space right above a combustion chamber to make it difficult to ensure an installation space for a spark plug, posing a problem that the layout of the spark plug, etc. is limited.
In the variable valve train apparatus shown in FIG. 20, the rollers 301a, 302a individually provided in the rocker arms 301, 302 for rolling motion on the corresponding cams are rocked according to the shapes of these cams. When the rollers make rocking motion according to the high-speed cam, the roller 301a on the low-speed rocker arm 301 does not achieve any function but exerts inertia mass in the direction to hinder the rocking motion of the rocker arm 301. As a result, the boss portion 301b of the low-speed rocker arm 301 is twisted in the forward and reverse directions each time the rocking motion is performed. Thus, the opening and closing characteristic of the intake valve based on the high-speed cam, especially, the opening and closing characteristic of the intake valve 304 that is driven to open or close by means of the valve arm portion 301c disposed on the side away from the high-speed rocker arm 302, is deviated from the intended one. This is one of the causes of lowering the opening and closing characteristic of the valve train in a high-speed rotation region.
In the variable valve train apparatus shown in FIGS. 21 and 22, the valve clearance is determined according to the positional relation of the low-speed rocker arm 404 with respect to the low-speed cam 401b and the intake valve 405 in the low-speed mode where the low-speed rocker arm 404 is directly rocked by the low-speed cam 401b. In the high-speed mode where the low-speed rocker arm 404 is indirectly rocked by the high-speed cam 401a by way of the high-speed rocker arm 402, the valve clearance is additionally affected by a combination of the low-speed rocker arm 404 and the high-speed rocker arm 402, so that a different valve clearance can be formed.
Therefore, even when the valve clearance on the side of the low-speed cam is adjusted to the normal one by means of the adjustment bolt 411 provided in the intake valve 405, this does not guarantee that an equivalent valve clearance can also be attained on the side of the high-speed cam. In order to ensure the proper valve clearance on the high-speed cam side after the assembly of the engine, the accuracy of individual component parts including the rocker arms 402, 404 is improved, and the shape of the high-speed cam 401a is designed in consideration of variations caused when these component parts are assembled (for instance, the high-speed cam 401a is configured to have an adequate ramp portion so as to relive impact on the roller 402a, or other countermeasure is taken).
Other than the shapes of the rocker arms 402 and 404, misalignment of the intake rocker shaft 403 may be mentioned as the factor affecting on the valve clearance on the side of the high-speed cam. Specifically, in a case where there is a vertical angular error α in the axis Lr of the intake rocker shaft 403 as shown in FIG. 22, the centers of rocking motion of the low-speed and high-speed rocker arms 404, 402 are relatively displaced from each other in the vertical direction, so that the relation between the valve clearance on the low-speed cam side and that on the high-speed cam side is varied. The same result is caused when a vertical angular error a in the axis Lc is produced due to misalignment of the camshaft 401.
Since the angular errors α of the intake rocker shaft 403 and the camshaft 401 are directly related to the error of the valve clearance, the aforementioned misalignments have a greater effect than that of other factors, which effect cannot be eliminated by the aforesaid countermeasure. As a consequence, when the valve clearance is so adjusted as to meet the low-speed mode, the proper valve clearance suitable for the high-speed mode cannot be attained, resulting in hammering sound. In addition, there occurs a problem of individual difference in engine valve clearance, making it difficult to attain uniform quality.
The variable valve train apparatus shown in FIGS. 23 and 24 requires the provision of three rocker arms 504 and 505 on the intake side and four rocker arms 508 and 508 on the exhaust side for every cylinder. In addition, seven cams 506, 507, 510 and 511 for rocking operation of the rocker arms 504, 505, 508 and 509 must be provided on the camshaft 501. This increases the number of component parts and man-hours for machining, resulting in a problem of increased fabrication cost. Furthermore, with the increase of the number of cams, the camshaft length per cylinder increases, which requires a large space, and the cylinder distance inevitably increases. This results in an oversized engine.